Method of depositing nanoscale materials within a nanofiber network and networked nanofibers with coating

ABSTRACT

Provided herein is a method of making a conductive network by combining uncoated carbon nanotubes and carbon nanotubes coated with an electroactive substance to create an electrically conductive network; and redistributing at least a portion of the electroactive substance. Also provided herein is an electrically conductive network with an active material coating; first carbon nanotubes coated with the active material coating; and second carbon nanotubes partially coated with the active material coating, wherein at least a portion of the surfaces of the second carbon nanotubes directly contact surfaces of other second carbon nanotubes without the active material coating between these second carbon nanotubes, and wherein the first carbon nanotubes and the second carbon nanotubes are entangled to form an electrically conductive network.

This application is a continuation of U.S. patent application Ser. No. 15/784,476, filed Oct. 16, 2017, which is a continuation application of U.S. patent application Ser. No. 14/774,752, filed Sep. 11, 2015, which issued as U.S. Pat. No. 9,793,548, which is a National Stage Entry of PCT Application No. PCT/US14/24458, filed on Mar. 12, 2014, which claims priority from U.S. Provisional Patent Application 61/786,776, filed on Mar. 15, 2013, all of which are hereby incorporated by reference in their entireties.

BACKGROUND

With the ever increasing use of batteries, consumers desire better performance in terms of speed of charging and discharging, as well as charge capacity from their batteries.

Carbon nanotubes (and other nanosized objects) are becoming more popular in manufacturing as supply increases. However, methods for coating carbon nanotubes have been limited to traditional coating techniques, which lead to non-uniform coating characteristics, especially when the carbon nanotubes are networked prior to coating.

SUMMARY

Due to the size of porous regions within nanofiber networks, deposition of materials within these porous or interstitial regions has been difficult post-networking of the nanofibers. A method of depositing nanoscale materials within a nanofiber network and networked nanofibers with coating are described herein.

Also provided herein is a method of manufacturing a nanoscale coated network, which includes providing nanofibers, capable of forming a network in the presence of a liquid vehicle; providing a nanoscale solid substance in the presence of the liquid vehicle; forming a network of the nanofibers and the nanoscale solid substance; and redistributing at least a portion of the nanoscale solid substance within the network, thereby producing a network of nanofibers coated with the nanoscale solid substance.

Also provided herein is a method of making a conductive network, which includes combining uncoated carbon nanotubes and coated carbon nanotubes coated with an electroactive substance to create an electrically conductive network; and redistributing at least a portion of the electroactive substance.

Also provided herein is an electrically conductive network, which includes an active material coating; first carbon nanotubes coated with the active material coating; and second carbon nanotubes partially coated with the active material coating, wherein at least a portion of the surfaces of the second carbon nanotubes directly contact surfaces of other second carbon nanotubes without the active material coating between these second carbon nanotubes, and wherein the first carbon nanotubes and the second carbon nanotubes are entangled to form an electrically conductive network.

Also provided herein is an electrically conductive, chemically insulated network of nanofibers, which includes first carbon nanofibers electrically connected to second carbon nanofibers to form an electrically conductive network, and second carbon nanofibers electrically connected to other second carbon nanofibers, wherein at least one of the second carbon nanofibers is in direct surface contact with another of the second carbon nanofibers; and an active material that provides electrochemical insulation on surfaces of the first carbon nanofibers and partial surfaces of at least a portion of the second carbon nanofibers, wherein the active material comprises at least 50% by weight of the electrically conductive, chemically insulated network.

Also provided herein is a nanoscale coated network, which includes one or more first carbon nanotubes electrically connected to one or more second carbon nanotubes to form a nanoscale network, wherein at least one of the one or more first carbon nanotubes is in electrical contact with one or more second carbon nanotubes; and an active material coating that covers at least a portion of the one or more first carbon nanotubes and at least a portion of the one or more second carbon nanotubes to form the nanoscale coated network, wherein the active material coating surrounds, but does not electrically disrupt the electrical contact between the one or more second carbon nanotubes.

Also provided herein is an electrically conductive, chemically insulated network of nanofibers, which includes one or more first carbon nanofibers electrically connected to one or more second carbon nanofibers to form an electrically conductive network, wherein at least one of the one or more second carbon nanofibers is in electrical contact with another of the one or more second carbon nanofibers; and an active material that provides electrochemical insulation on an outer portion of at least a portion of the one or more first carbon nanofibers and at least a portion of the one or more second carbon nanofibers, wherein the active material comprises at least 50% by weight of the electrically conductive, chemically insulated network.

Also provided herein is a coated nanofiber network, which includes one or more first carbon nanotubes; one or more second carbon nanotubes, wherein at least one of the one or more second carbon nanotubes is in electrical contact with another of the one or more second carbon nanotubes to form a carbon nanotube network; and an active material that covers at least a portion of the carbon nanotube network to form the coated carbon nanotube network, wherein the active material coating surrounds, but does not interfere with the electrical contact between the one or more second carbon nanotubes.

Also provided herein is a coated nanofiber network, which includes one or more first carbon nanotubes; one or more second carbon nanotubes, wherein at least one of the one or more second carbon nanotubes is in electrical contact with another of the one or more second carbon nanotubes to form a carbon nanotube network; and a coating comprising nanoscale compounds of Ni, Zn, Cd, Fe, Pb, Mn, Co, Ag, Al, or Mg that covers at least a portion of the carbon nanotube network to form the coated carbon nanotube network, wherein the one or more first carbon nanotubes and the one or more second carbon nanotubes comprise at most 50% by weight of the coated carbon nanotube network, and the coating comprises at least 50% by weight of the coated carbon nanotube network.

Also provided herein is a coated nanofiber network, which includes one or more first carbon nanotubes; one or more second carbon nanotubes, wherein at least one of the one or more second carbon nanotubes is in electrical contact with another of the one or more second carbon nanotubes to form a carbon nanotube network; and a coating comprising nanoscale particles that cover at least a portion of the carbon nanotube network to form the coated carbon nanotube network, wherein the coated carbon nanotube network has a volume of porosity of 50-90 vol. %.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated and constitute a part of this specification, illustrate an embodiment of the invention. In the drawings,

FIG. 1 is an overview illustration of a bulk volume with nanofibers and thick fibers used as support for an active material;

FIG. 2 is an overview illustration of a method by which an electrode may be formed;

FIGS. 3A-3F are example illustrations for forming an electrode by the method of FIG. 2;

FIG. 3G is an example illustration for forming a battery using the electrode from the method of FIG. 2;

FIG. 4 is a Scanning Electron Microscope (SEM) image of a network of nanofibers;

FIG. 5 is an SEM image of nanofibers including active material thereon prior to redistribution of the active material within the nanofibers;

FIG. 6 is an SEM image of nanofibers including active material thereon after redistribution of the active material within the nanofibers;

FIG. 7 is an example flowchart of a step of the example method of FIG. 2;

FIGS. 8A-8B are example illustrations of nanofibers and active material when the active material is redistributed among the nanofibers; and

FIG. 9 is an example graphical illustration of charge-discharge results for a nanofiber-nanoscale active material electrode before and after redistribution of the active material.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description describes embodiments and is not intended to limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents.

A. Overview

Provided herein are electrodes that can provide high speed, high capacity, light weight, and safety in batteries. These electrodes can utilize properties of nanofibers and nanoscale active materials, in conjunction with a current collector, to increase the speed and capacity without additional weight and/or additional safety concerns.

FIG. 1 depicts a bulk volume with nanofibers 110 and thicker fibers 120 used as supports for an active material. As illustrated, thin layers of active material on nanofibers 110 provide more capacity than thin layers of active material on thicker fibers 120 in the same bulk volume.

Increasing the energy density of the active material can also be accomplished by distributing the active material throughout the conductive support network. In one implementation, increasing the energy density of active material can be done by coating a conductive support system with active material. For example, as illustrated in FIG. 2, a fast fibril electrode can be provided in one embodiment by example method 200.

B. Method of Making an Electrode

FIG. 2 illustrates an example method 200 by which an electrode may be formed.

FIGS. 3A-3F are example illustrations for forming an electrode by method 200 of FIG. 2. FIG. 3G is an example illustration for forming a battery using the electrode from method 200 of FIG. 2.

Method 200 can include providing nanofibers in step 210. In one implementation, the nanofibers can be provided in a liquid medium or in another media, such as air.

For example, as illustrated in FIG. 3A, first nanofibers 310 and second nanofibers 315 can be provided in a first vessel 320. Nanofibers 310, 315 may be the same or different sized (i.e., diameter, length/diameter ratio, agglomeration size, etc.) nanofibers and may be formed of the same or different types of materials within nanofibers 310 and nanofibers 315, respectively, or between nanofibers 310 and nanofibers 315. For example, nanofibers 310 and nanofibers 315 may be single wall or multi-wall nanotubes, and may further include additional microfibers and/or macrofibers. In one embodiment, one or more nanofibers in nanofibers 315 may include the same or different nanofibers (and microfibers and/or macrofibers) compared to one or more nanofibers in nanofibers 310 or compared to one or more nanofibers in other nanofibers 315.

Additionally, nanofibers 310, 315 may be produced in the same or different batches, which may also yield variations in size, shape, or structure. In one embodiment, nanofibers 310 and nanofibers 315 are similar within each group of nanofibers 310 and nanofibers 315, respectively. In one embodiment, nanofibers 310 and nanofibers 315 may be similar in size and shape throughout nanofibers 310 and nanofibers 315. The various shapes, sizes, and structures for nanofibers 310, 315 are further discussed below.

Nanofibers 310, 315 may be provided in liquid medium 325 that can allow nanofibers 310, 315 to self-assemble (i.e., aggregate or agglomerate) or remain independent (i.e., maintain a certain spacing distance) from other nanofibers 310, 315. In one embodiment, liquid medium 325 can include a liquid vehicle, such as an aqueous solution or an electrolyte. For example, liquid medium 325 may be water. Further discussion on networking of nanofibers may be found in U.S. Pat. Nos. 6,099,965, 7,923,403, and U.S. Patent Application Publication No. 2008/0176052 A1, which are incorporated herein in their entireties.

FIG. 4 is a Scanning Electron Microscope (SEM) image of a network of nanofibers. As illustrated in FIG. 4, nanofibers 310, 315 may be networked or entangled to form one or more aggregations. Further discussion of nanofibers 310, 315 follows below. It is noted that at least some of nanofibers 310, 315 may preferably be electrically conductive.

Method 200 can also include depositing an active material on first nanofibers 310 to form coated nanofibers in step 220. The active material, as discussed further below, may be any material capable of providing an acceptable energy density and potential for a battery electrode, such as a chemically active nanoscale solid substance, as further discussed below. In one implementation, deposition of the active material may occur by separating first nanofibers 310 from second nanofibers 315, such that only first nanofibers 310 (or second nanofibers 315) may be subjected to the deposition of the active material to form coated nanofibers, while second nanofibers 315 may remain non-coated nanofibers. While the active material is deposited on first nanofibers 310 to coat nanofibers in step 220, other materials, such as nanoscale substances may also be deposited on first nanofibers 310. For example, other nanoscale substances may also be deposited, as further discussed below.

For example, as illustrated in FIG. 3B, first nanofibers 310 can be placed in second vessel 340, while second nanofibers 315 can be placed in third vessel 350. Active material 330 may be deposited on first nanofibers 310 in second vessel 340, while nanofibers 315 in third vessel 350 may remain free of active material 330. Deposition of active material 330 can be done by any method that allows active material 330 to adhere to a surface of nanofibers 310. For example, deposition can occur in a liquid phase by chemical or electrochemical deposition. As another example, deposition can occur in a gas phase by chemical vapor deposition or physical vapor deposition. In one implementation, the active material 330 may include a chemically active nanoscale solid substance, such as one or more of hydroxides, carbonates, fluorides, sulfates, oxalates, phosphates of one or more compounds, such as Ni, Zn, Cd, Fe, Pb, Mn, Co, Ag, Al, or Mg.

Method 200 can also include combining first nanofibers 310 coated with active material 330 (from step 210 or the like) and second nanofibers 315, which are not yet coated, in step 230. In one implementation, first nanofibers 310 with active material 330 coating and second nanofibers 315 may be combined by physically mixing them together in a liquid vehicle. For example, they can be mixed by any means, such as by using a mixer, an agitator, a sonicator, or an ultrasonicator. In another implementation, they can be mixed in a dry state by any means, such as a mixer, a blender, or a mill, where the mill can mix them by milling the active materials and the nanofibers together in any kind of high intensity device, including, but not limited to a ball mill or rod mill, colloid mill or microfluidizer in a continuous or a batch operation.

For example, as illustrated in FIG. 3C, nanofibers 310 with coating 330 from second vessel 340 may be combined with non-coated nanofibers 315 from third vessel 350.

In one example, as illustrated in FIG. 3D, first nanofibers 310 with coating 330 and second nanofibers 315 (without coating 330) can be networked together to form an electrically conductive network of nanofibers with electrical communication areas 360 and electrical contacts 365. By combining first nanofibers 310 with active material 330 and second nanofibers 315, electrical conductivity between first nanofibers 310 and second nanofibers 315 can be provided within electrical communication areas 360, and between second nanofibers 315 and other second nanofibers 315 within electrical contacts 365. It is noted that the electrical contacts 365 between second nanofibers 315 will not be hindered by active material 330, and can be in direct surface contact. This direct surface contact can be maintained because second nanofibers 315 can entangle with other second nanofibers 315 without either first nanofibers 310 or active material 330 being between second nanofibers 315. Also, because of a plurality of these electrical contacts 365, the overall network can be very conductive. By redistributing at least a portion of active material 330 after network formation from coated first nanofibers 310 with active material 330 thereon to uncoated second nanofibers 315, active material 330 can be moved from surfaces of first nanofibers 310 to second nanofibers 315 without interfering with electrical contacts 365 between second nanofibers 315.

For example, as illustrated in FIG. 5, which is an SEM image of nanofibers including active material thereon prior to redistribution of the active material within the nanofibers, a network of first nanofibers 310 with active material 330 (i.e., nanofibers 310 with active material 330 located on the surface) and second nanofibers 315 (i.e., nanofibers without active material 330) may be provided. As shown in FIG. 5, active material 330 may be present on first nanofibers 310 and not present on second nanofibers 315 (and not present between second nanofibers 315 within electrical contacts 365).

Method 200 can include redistributing active material 330 throughout the network in step 240. In one implementation, redistribution of active material 330 can be provided by recrystallizing active material 330 from active material 330 on the surface of first nanofibers 310 to the surface of second nanofibers 315. In another implementation, redistribution of active material 330 may be provided via electrical charge and discharge.

For example, as illustrated in FIG. 3E, active material 330 from coated first nanofibers 310 can be redistributed onto a portion of the previously non-coated second nanofibers 315 to redistribute active material 330. Advantageously, in one implementation, active material 330 from coated first nanofibers 310 can provide coverage of the surface of coated first nanofibers 310 and coverage of the surface of the previously non-coated second nanofiber 315 such that electrolytes that may be brought into contact with coated first nanofibers 310 and the previously non-coated second nanofiber 315 would not contact either of the underlying nanofibers 310, 315. As mentioned above, non-coated second nanofibers 315 can maintain non-coated electrical contacts 365 where active material 330 is not between second nanofibers 315.

By providing active material 330 directly on nanofibers 310, 315, electrical conduction paths between nanofibers 310, 315 and active material 330 can be reduced in length, thus leading to increased electrical conduction speed through a resulting electrode.

In one implementation, as illustrated in FIG. 6, which is an SEM image of nanofibers including active material thereon after redistribution of the active material within the nanofibers, active material 330 from surfaces of coated first nanofibers 310 can move to cover large areas of second nanofiber 315 with deposits by distributing active material 330 from first coated nanofibers 310 and to previously non-coated second nanofibers 315.

One implementation is illustrated in FIG. 7, which is an example flowchart of step 240 of method 200 of FIG. 2, and in FIG. 8A, which is an example illustration of nanofibers 310, 315 and active material 330 when active material 330 is redistributed among the nanofibers 310, 315. In FIGS. 7 and 8A, redistributing active material 330 can occur by providing an electrical charge to nanofiber network 810, which includes first (i.e., coated) nanofibers 310 and second (i.e., non-coated) nanofibers 315, in sub-step 710 of step 240. By providing an electrical charge, some of active material 330 can be released from first nanofibers 310 as released active material 820. Released active material 820 can be released into areas among coated first nanofibers 310 and non-coated second nanofibers 315.

Redistribution of released active material 820 from first nanofibers 310 to second nanofibers 315 can occur in sub-step 820. It is believed that released active material 820 will deposit onto the most chemically active areas of second nanofibers 315, which should be the non-coated surfaces of second nanofibers 315. It is also believed that while the non-coated areas of second nanofibers 315 are coated with released active material 820, the newly coated areas will become less chemically active and less attractive to released active material 820 compared to non-coated areas of second nanofibers 315. Eventually, the non-coated areas of second nanofibers 315 can become coated and the electrochemical activity of first nanofibers 310 and second nanofibers 315 should reach equilibrium from the redistribution of released active material 820.

Alternatively, other mechanisms can be used to release active material 330 from coated first nanofibers 310, as released active material 820 for deposition on second nanofibers 315. For example, the network of coated nanofibers 310 with active material 330 and uncoated nanofibers 315 can be subjected to repeated heating and cooling cycles in a liquid vehicle. Active material 330 can be partially released as released active material 820 during a heating cycle and re-deposited in different locations during the cooling cycle.

Next, as illustrated in FIG. 8B, which is an example illustration of nanofibers 310, 315 and active material 330 when active material 330 is redistributed among nanofibers 310, 315, nanofiber network 810, which can be an agglomeration or aggregation of first nanofibers 310 and second nanofibers 315, can be made accessible to released active material 820. A coated nanofiber network 830 may be formed by allowing released active material 820 to attach to non-coated second nanofibers 315 (and coated first nanofibers 310) to form coated nanofiber network 830 via redistribution. In one implementation, several redistributions of active material 330 via various redistribution mechanisms may occur to allow released active material 820 to coat previously non-coated second nanofibers 315. One example of a redistribution mechanism may be recrystallization.

By redistributing active material 330 to form coated nanofiber network 830, electrical communication between second nanofibers 315 can be established within coated nanofiber network 830. This can occur before second nanofibers 315 are coated to allow electrical communication between second nanofibers 315 to be preserved. By preserving the electrical communication, electrical conductivity between second nanofibers 315 can be uninterrupted by active material 330, yet active material 330 coverage throughout coated nanofiber network 830 can be optimized.

Coating first nanofibers 310 and second nanofibers 315 prior to networking may cause active material 330 to insulate junctions between nanofibers 310, 315 and may prevent nanofibers 310, 315 from having electrical communication with one another. As such, the network formation step is preferably before the coating or at least before the completion of the coating step (e.g., before redistribution is complete). By coating first nanofibers 310 prior to networking, while not coating second nanofibers 315 prior to networking, direct surface contact can be created between second nanofibers 315 without any active material 330 between their surfaces. Additionally, by redistributing active material 330 after network formation, the direct surface contact between second nanofibers 315 can be maintained and thus electrical communication can be preserved beneath a surface of active material 330 that can be provided during redistribution.

It is noted that networks of nanofibers 310, 315 can be tightly bound in that interstitial spaces are formed between nanofibers 310, 315. These interstitial spaces are too small for conventional coating techniques, such as dip coating, spray coating, or the like, due to surface tension and the small nanoscale sizes of nanofibers 310, 315 not allowing a coating to penetrate the nanoscale size spaces between nanofibers 310, 315. Thus, conventional coating techniques after network formation can coat outer portions of the networks but are unable to breach or penetrate the interstitial spaces with regularity due to size requirements associated with conventional coating techniques. As illustrated in FIG. 4 and FIG. 5, these interstitial spaces between nanofibers 310, 315 can have a maximum dimension of less than 200 nm. FIG. 4 and FIG. 5 also illustrate active material 330 within the interstitial spaces, as well as on surfaces of nanofibers 310, 315.

FIG. 9 is an example graphical illustration of charge-discharge results for an example nanofiber-nanoscale active material electrode before and after redistribution of active material 330. As illustrated in FIG. 9, nanofibers which are divided, coated, networked, and redistributed show superior charge and discharge properties compared to nanofibers which are coated prior to networking. In FIG. 9, two similar samples, a first sample with nanofibers coated with ZnCO₃, a second sample with ⅔ of the nanofibers coated with ZnCO₃, mixed with ⅓ non-coated nanofibers. Both samples were charged and discharged at 2C rate (the currents, at which the battery is expected to be charged and discharged in ½ hour). FIG. 9 shows a cycle in 30% K₂CO₃ saturated with ZnO electrolyte. One can see that the charge curve is noisy for the first sample, pointing to poor electrical contacts. The second sample, on the other hand, is smooth. This result, in addition to the higher potentials on the discharge curve of the second sample, appears to indicate that the second sample has better electrical contacts (lower internal resistance).

Method 200 can include forming an electrode from coated nanofiber network 830 in step 250. In one implementation, coated nanofiber network 830 can be wetted with an electrolyte. Next, wetted coated nanofiber network 830 can be made into a paste, and the paste can be formed into an electrode. For example, the paste may be pressed onto a current collector, such as a conductive film, current collector plate, etc. In another implementation, coated nanofiber network 830 can be its own current collector.

Additionally, as illustrated in FIG. 3F, a paste of coated nanofiber network 830 (or other coated nanofiber network) can be provided on a current collector plate 370 and a lead 380 can be attached to form an electrode 390. In one implementation, coated nanofiber network 830 may be wetted with the same or different electrolyte as the electrolyte used in networking first nanofibers 310 and second nanofibers 315 in step 240. Additionally, step 240 and step 250 may be done in any order, such as forming the electrode in step 250, then redistributing the active material in step 240; redistributing the active material in step 240, then forming the electrode in step 250; concurrently forming the electrode and redistributing the active material in steps 240 and 250; or may include additional intervening steps.

Method 200 can include repeating steps 210 to 250 to provide additional electrodes. In one implementation, steps 210 to 250 can be done to form an anode, then using a different active material, can be repeated to form a cathode.

FIG. 3G is an example illustration for forming a battery using the electrode from method 200 of FIG. 2. For example, as illustrated in FIG. 3G, two electrodes 390 with leads 380 can be placed in a vessel 385 with electrolyte 395 to form a battery.

C. Electrolyte Selection

One way of increasing the charging/discharging speed of batteries is to utilize a fast electrolyte. Aqueous electrolytes can be safe and fast, but can have limited usefulness as water decomposes at about one volt. Non-aqueous electrolytes can tolerate higher voltages, but are usually less conductive (i.e. slower) than aqueous electrolytes as well as having safety issues, such as flammability and explosiveness. In one embodiment, the electrolyte may include an aqueous, ionically conductive electrolyte.

An electrolyte can be selected based upon compatibility with the electrodes selected. Many battery chemistries, especially those with metal anodes, also suffer from slow dissolution of active materials into the electrolyte and/or slow self-discharge by undesirable side reactions. Although methods to mitigate these are known, the degree of the problem is basically proportional to the surface area of the active material. Inasmuch as nanofiber electrodes have much higher surface area than most battery electrodes, many known methods prove impracticable for nanofiber based electrodes. For example, the quantity of additive, mitigant, etc. can exceed the solubility or become otherwise impractical for use.

Aqueous electrolytes, as mentioned above, can suffer from undesirable side reactions causing gas evolution. Exposed surface of the conductive support network may catalyze hydrogen evolution or oxygen evolution from water at more or less the same voltages needed to cause the desired half-cell reactions. Thus, surfaces of the conductive support network may be desirably chemically insulated by covering with active material to reduce gas evolution and increase charge-discharge efficiency to chemically insulate the nanofibers using the active material to reduce interaction between the nanofibers and chemical reactions with electrolytes.

Depositing additional active material may not achieve coverage of the conductive support network to reduce or prevent gas evolution. In one embodiment herein, active material 330 may be provided such that active material 330 chemically isolates first nanofibers 310 and second nanofibers 315, preferably when networked as coated nanofiber network 830, from electrolytes. It is believed that depositing more active material 330 may not achieve complete coverage. Crystallization or deposition from solution would preferentially deposit active material 330 on active material 330 that is already there rather than improving coverage of the remaining bare surfaces of nanofibers 310, 315. The energy of crystal formation on the same material is usually lower than nucleation on a different material, thus the already deposited active material 330 can serve as a more preferable substrate for the further deposition of additional material 330.

D. Nanofiber Selection

As the active materials generally do not provide much, if any mechanical support, nanofibers in the form of a network of nanofibers can be provided for mechanical support of the active materials in an electrode. In addition to providing a support for the active materials, the network of nanofibers can also be used to conduct electricity (i.e., provide a pathway for electrons) from electrodes to an outside load. A network of nanofibers can be formed by aggregating nanofibers into a random interpenetrating network, which can provide a pathway for electrons to access active material supported by the random interpenetrating network. As mentioned above, at least some of the nanofibers are preferably electrically conductive.

Additionally, in order for the battery to be fast, both in charge and discharge, some of the active material can be provided in close proximity with a nanofiber (i.e., in contact). By providing a network of nanofibers to support the active materials, the distance between the active material and a nanofiber in the network of nanofibers can approach zero to allow electrons to flow between the active material and the nanofiber readily.

The terms “nanotube,” “fibril,” and “carbon nanotube” are used interchangeably to refer to single wall (i.e., only a single graphene layer parallel to the nanotube axis) or multi-wall (i.e., more than one graphene layer more or less parallel to the nanotube axis) carbon nanotubes or other nanoscale sized fibers. Each refers to an elongated structure having a cross-section (e.g., angular fibers having edges) or a diameter (e.g., rounded) of, for example for multi-wall nanotubes, less than 1 micron, less than 0.5 microns, less than 0.2 microns, less than 100 nm, less than 50 nm, less than 20 nm; or for example for single wall nanotubes, less than 5 nanometers. Other types of carbon nanotubes are also known, such as fishbone fibrils (e.g., wherein the graphene sheets are disposed in a herringbone pattern with respect to the nanotube axis), “buckytubes,” etc. As produced, carbon nanotubes may be in the form of discrete nanotubes, aggregates of nanotubes (i.e., dense, microscopic particulate structure comprising entangled carbon nanotubes) or a mixture of both. Each of these conformations and structures may be used as “nanofibers” as discussed herein, as each would provide electrically conductive, networkable structures to support active materials.

The term “nanofiber” is broader, encompassing both nanotubes and other nanosized fibers that may not be hollow. The nanofiber may be oriented such that orientation(s) of graphenic sheet(s) may be at an angle to the axis (including perpendicular) of the nanofiber or may lack the defined orientation of the graphenic sheets or which may be covered with an outer layer of pyrolytic carbon. Many of these structures whose wall structure is not parallel to the fiber axis may have very small hollow cores or a hollow core that may not be discernible. It is not necessary that these nanofibers be of circular cross section although symmetrical cross sections are preferred. Nanofibers that have been graphitized in a separate step post synthesis may be used. The graphitization may be partial or complete as measured by the temperature employed. Further discussion on nanofibers can be found in U.S. Pat. Nos. 5,800,706 and/or 6,099,960, which are incorporated in their entireties by reference.

Nanofibers exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces. These include those described in U.S. Pat. No. 6,099,965 to Tennent, et al. and U.S. Pat. No. 5,569,635 to Moy, et al., both of which are hereby incorporated by reference in their entireties.

In an embodiment, nanofibers are made by catalytic growth from hydrocarbons or other gaseous carbon compounds, such as CO, mediated by supported or free floating catalyst particles. Preferably, catalysts used for catalytic growth are supported on inert supports such as silica, alumina or magnesia. Preferably, supported or free floating catalyst particles are not removed prior to forming the nanofiber network. In some cases, however, it may be desirable to remove the catalyst particles and this may be done by chemical treatment with acids or bases or thermally (e.g., microwave treatment).

Nanofibers may also be formed as aggregates of carbon nanotubes, which may be dense microscope particulate structures of entangled carbon nanotubes and may resemble the morphology of bird nest (“BN”), cotton candy (“CC”), combed yarn (“CY”) or open net (“ON”). Nanofibers may also be grown on a flat support, attached by one end to the support and parallel to each other, forming a “forest” structure. Aggregates are formed during the production of carbon nanotubes and the morphology of the aggregate is influenced by the choice of catalyst support. Porous supports with completely random internal texture, e.g., fumed silica or fumed alumina, grow nanotubes in all directions leading to the formation of bird nest aggregates. Combed yarn and open net aggregates are prepared using supports (e.g., alumina or magnesia) having one or more readily cleavable planar surfaces, e.g., an iron or iron-containing metal catalyst particle deposited on a support material having one or more readily cleavable surfaces and a surface area of at least 1 square meter per gram.

Individual carbon nanotubes in aggregates may be oriented in a particular direction (e.g., as in “CC,” “CY,” and “ON” aggregates) or may be non-oriented (i.e., randomly oriented in different directions, for example, as in “BN” aggregates). Carbon nanotube “agglomerates” are composed of carbon nanotube “aggregates.” Carbon nanotube “aggregates” retain their structure in the carbon nanotube “agglomerates.” As such, “BN” agglomerates, for example, may contain “BN” aggregates.

“BN” structures may be prepared as disclosed in U.S. Pat. No. 5,456,897, for example, which is hereby incorporated by reference in its entirety. “BN” agglomerates are tightly packed with typical densities of greater than 0.1 g/cc, for example, 0.12 g/cc. Transmission electron microscopy (“TEM”) reveals no true orientation for carbon nanotubes formed as “BN” agglomerates. Patents describing processes and catalysts used to produce “BN” agglomerates include U.S. Pat. Nos. 5,707,916 and 5,500,200, both of which are hereby incorporated by reference in their entireties.

On the other hand, “CC,” “ON,” and “CY” agglomerates have lower density, typically less than 0.1 g/cc, for example, 0.08 g/cc and their TEMs reveal a preferred orientation of the nanotubes. U.S. Pat. No. 5,456,897, hereby incorporated by reference in its entirety, describes the production of these oriented agglomerates from catalyst supported on planar supports (i.e. those having one or more readily cleavable planar surfaces of alumina or magnesia, for example).

“CY” may also refer generically to aggregates in which the individual carbon nanotubes are oriented, with “CC” aggregates being a more specific, low density form of “CY” aggregates.

Carbon nanotubes are distinguishable from commercially available continuous carbon fibers. For instance, the diameter of continuous carbon fibers, which is always greater than 1.0 micron and typically 5 to 7 microns, is also far larger than that of carbon nanotubes, which is usually less than 1.0 micron. Carbon nanotubes also have vastly superior strength and conductivity than carbon fibers.

Carbon nanotubes also differ physically and chemically from other forms of carbon such as standard graphite and carbon black. Standard graphite is, by definition, flat. Carbon black is an amorphous structure of irregular shape, generally characterized by the presence of both sp2 and sp3 bonding. On the other hand, carbon nanotubes have one or more layers of ordered graphitic carbon atoms disposed substantially concentrically about the cylindrical axis of the nanotube. These differences, among others, make graphite and carbon black poor predictors of carbon nanotube chemistry.

“Multi-wall nanotubes” as used herein refers to carbon nanotubes which are substantially cylindrical, graphitic nanotubes of substantially constant diameter and comprise cylindrical graphitic sheets or layers whose c-axes are substantially perpendicular to the cylindrical axis, such as those described, e.g., in U.S. Pat. No. 5,171,560 to Tennent, et al. The term “multi-wall nanotubes” is meant to be interchangeable with all variations of said term, including but not limited to “multi-wall nanotubes,” “multi-walled nanotubes,” “multiwall nanotubes,” etc.

“Single wall nanotubes” as used herein refers to carbon nanotubes which are substantially cylindrical, graphitic nanotubes of substantially constant diameter and comprise a single cylindrical graphitic sheet or layer whose c-axis is substantially perpendicular to the cylindrical axis, such as those described, e.g., in U.S. Pat. No. 6,221,330 to Moy, et al. The term “single wall nanotubes” is meant to be interchangeable with all variations of said term, including but not limited to “single-wall nanotubes,” “single-walled nanotubes,” “single wall nanotubes,” etc.

Multi-wall nanotubes as used herein subsumes the term “few-walled nanotubes.” Such tubes having only two or three walls occupy a niche between single wall and multi-wall nanotubes, but may nevertheless be useful in the practice described herein.

It is understood that multi-wall carbon nanotubes may be readily functionalized. Methods of functionalizing nanotubes are discussed in U.S. Pat. Nos. 6,203,814, 7,413,723, and 6,872,681, which are incorporated by reference in their entireties. Such functionalized multi-wall carbon nanotubes may be more readily dispersed in aqueous media than as-made, non-functionalized multi-wall carbon nanotubes. Either functionalized or as-made nanotubes may be used herein.

Oxidation may be a first step in functionalization and may be done with either liquid phase or gas phase reagents. An initial oxidation may be followed by additional chemical reactions to convert the initial oxidation created functionalities to other chemical moieties. The functionalization may yield either a uniform chemical functionality or a mixture of chemical functionalities.

Adsorption may be another potential first step in functionalization. Compounds having aromatic or polyaromatic functionality may adsorb strongly onto carbon nanotubes. Porphyrins and aromatic acids are examples of such molecules.

Generally, functionally modified nanotubes may aggravate side reactions of an aqueous electrolyte with the functionally modified nanotubes. However, the functional groups on the surface may be beneficial for better adherence of active material 330 to a nanofiber. In one implementation, the step of redistributing active material on coated nanofiber network 830 may be helped by functional groups. After the redistribution, the nanofibers will no longer be in direct contact with electrolyte (as the nanofibers will be coated); therefore side reactions may be minimized. The conductivity of nanofiber networks depends not only on the inherent conductivity of the nanofibers, but also upon the average length and spatial density of the fibers in the network. Network resistance is believed to derive mainly from the fiber-fiber resistance at the intersections.

E. Active Material Selection

The terms “active material,” “electroactive substance,” and “electroactive agent” are used interchangeably to refer to chemical compounds that provide chemical energy for conversion to electrical energy in a battery. The active material may be a chemically active material in that it may be a substance that can participate in the release or acceptance of an electron. The active material may also be provided on a nanoscale level. In one embodiment, the active material may be chemically active nanoscale solid substances, such as nanoscale sized particles of chemically active material.

The choice of active material for a battery depends on factors other than energy density and power density. These include, but are not limited to: cost, safety, life, reliability, temperature stability, failure mode, etc. In embodiments provided herein, electrodes are provided that can improve the power density of any battery system or individual electrode. Electrode chemistries known to be reversible are, however, preferred. These include, but are not limited to NiOOH/Ni(OH)₂; Zn/ZnOH; Cd/Cd(OH)₂; Fe/Fe(OH)₂; Pb/Pb(OH)₂; Pb/PbSO₄; MnO₂/Mn₂O₃; PbO₂/PbSO₄; Co/Co(OH)₂; Ag/AgO; Al/Al₂O₃; Mg/Mg(OH)₂, Metal/Metal Hydride; etc.

In one embodiment, the active material may be provided by depositing the active material in nanoscale sized form from solution. In one embodiment, the active material may be nanoscale solid material after deposition on a nanofiber.

Additionally, the active material, upon application, may provide insulation from an electrolyte in an electrode, as described herein. In one embodiment, the active material may reduce or prevent interaction between the nanofibers and electrolyte in an electrode. For example, by utilizing the methods provided herein, side reactions between the nanofibers and the electrolyte may be reduced by the presence of the active material insulating the nanofibers from the electrolyte.

Systems compatible with an aqueous electrolyte are also preferred because aqueous electrolytes can better exploit the energy density of the batteries described herein.

F. Electrode Formation

In embodiments herein, electrodes can be made or provided in the form of a two dimensional sheet or mat. If a two dimensional sheet is provided, then the sheet may be assembled into a device with a current collector. For example, a current collector can be provided in the form of a foil or conductive layer aligned in parallel to the electrode and in intimate contact therewith. The through sheet conductivity of the electrode must be high enough not to limit the power density of the battery.

The electrode may also include non-conductive structural components, such as non-conductive nanofibers. The concentration of such structural components is not critical so long as the electrode conductivity is not compromised.

If a three dimensional mat is provided, then the mat may have a thickness as desired. While the performance of the electrode may vary with the thickness of the active material, such variations may also occur based upon different active materials.

In one embodiment, a coated nanotube network electrode can function as its own current collector. In this case, the coated nanotube network electrode can be connected to an outside load (or to other cells in the stack) through its edges, which causes the conductivity in the direction of the electrode plane (the x-y conductivity) to become critical to the cell resistance. This cell resistance may be less 200 ohms-cm, more preferably less than 100 ohms-cm, and still more preferably less than 50 ohms-cm.

G. Embodiments

In one embodiment, a nanofiber-MnO₂ electrode can be paired with a nanofiber-Zn electrode to provide a pair of fast fibril (nanofiber) electrodes in a battery. In other embodiments, electrodes with nanofibers and compounds of Zn, Co, Cd, Fe, and/or Pb can be paired with electrodes with nanofibers and compounds of Ni to provide fast fibril electrodes.

In one embodiment, a nanofiber electrode may contain more active material than nanofibers to allow for more active material to be present in an electrode. For example, a nanofiber electrode may contain less than 50 wt. % nanofibers. In another example, a nanofiber electrode may contain less than 25 wt. % nanofibers, which may also include more than 75 wt. % active material.

In one embodiment, a nanofiber electrode can have a porosity level that allows enough electrolyte to complete charge and discharge without precipitation. For example, a nanofiber electrode may contain a network with a volume of porosity of 50 to 90 vol. %, which can allow for sufficient levels of electrolyte to complete charge and discharge without precipitation. As another example, a nanofiber electrode may contain a network with a volume porosity of 50 to 80 vol. % to increase the electrode volume devoted to active ingredients, as porosity reduces electrode volume that could be devoted to active material.

It should be recognized that embodiments herein are describing the electrical and chemical aspects of the preferred electrodes. Other components may be added to the paste or mat for the electrodes to alter physical or electrical properties. Binders, additives to improve conductivity, cycle life, thermal stability, charge retention, shelf life, structural integrity, or other parameters may be employed. Generally, the quantity of additives should be small enough not to materially alter the energy or power density of the electrodes. For example, additives may preferably be added at less than 20 wt. % of the electrode, more preferably less than 10 wt. % of the electrode, and even more preferably less than 5 wt. % of the electrode. Examples of additives can be found in U.S. Pat. No. 6,790,559 (e.g., additives to Ni electrode: fluoride salts 0.1-1% for active material utilization) and U.S. Pat. No. 6,811,926 (e.g., additives to Zn electrodes: inorganic fibers (alumina and silica, 2-15%) and bismuth oxide (2-10%) for cycle life), both of which are incorporated by reference in their entireties.

H. Example—Method of Making an Electrode

Initially, active material 330 can be introduced into a network forming step by a prior “rough” deposition onto only a fraction of the nanofibers, in an aqueous electrolyte whose anion forms only a sparingly soluble salt, NiCO₃ or ZnCO₃, for example, with a cation of active material 330. A sparingly soluble salt may be any salt with solubility less than 1 g/100 g, but greater than zero. In one embodiment, the sparingly soluble salt may have solubility greater than zero and less than 0.1 g/100 g. For example, the sparing soluble salt may include, but is not limited to hydroxides, carbonates, fluorides, sulfates, oxalates, phosphates.

The sparingly soluble salt may be provided as an intermediate to assist in any redistribution of coating. If the sparingly soluble salt has been prior deposited onto a fraction of the nanofibers, then the network forming step can include both “roughly coated” and “plain” (non-coated/un-deposited) nanofibers.

Using method 200 described above, first nanofibers 310, preferably non-oxidized nanofibers, can be provided in a liquid vehicle that may include a readily soluble salt, such as Ni(NO₃)₂ or ZnSO₄, for example. Non-oxidized or oxidized fibers may be used. Further discussion of non-oxidized and oxidized fibers can be found in U.S. Pat. No. 7,413,723, which is incorporated in its entirety by reference.

A readily soluble salt as used herein may be any soluble compound that can form a sparingly soluble compound of the desirable chemistry. Chlorides, nitrates, bicarbonates, some sulfates, and other soluble salts may be used for the step of deposition of active material 220 of method 200. Next, a reactant, such as K₂CO₃ or KOH, can be added to the liquid vehicle, which includes a soluble salt, and the reactant can combine with the soluble salt to deposit the corresponding sparingly soluble salt on coated nanofibers 310. This sparingly soluble salt can become active material 330 after step 240 in method 200, discussed above.

Next, a repeated charge and discharge may be applied to a network of coated nanofibers 310 and non-coated nanofibers 315 in an appropriate electrolyte to redistribute active material 330 over all the nanofibers 310, 315 to form a coated nanofiber network 830.

Electrodes described herein may be used in batteries. The electrodes can be provided in single-use, non-rechargeable batteries (often referred to as “primary batteries”) or multiple use, rechargeable batteries (often referred to as “secondary batteries”). The electrodes can also be provided in flexible batteries, or other types of batteries.

While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to those skilled in the art that variations and modifications can be made, and equivalents employed without departing from the scope of the appended claims. 

What is claimed is:
 1. A method of making a conductive network, comprising the steps of: (a) combining to entangle uncoated carbon nanotubes and coated carbon nanotubes coated with an electroactive substance to create an electrically conductive network; and (b) redistributing at least a portion of the electroactive substance.
 2. The method of claim 1, wherein the combining to entangle uncoated carbon nanotubes and the coated carbon nanotubes coated with an electroactive substance to create an electrically conductive network comprises: entangling the uncoated carbon nanotubes and the carbon nanotubes with the electroactive substance such that the entanglement creates an electrically conductive network of uncoated carbon nanotubes entangled with coated carbon nanotubes coated with the electroactive substance.
 3. The method of claim 1, wherein the electroactive substance comprises a chemically active nanoscale solid substance.
 4. The method of claim 1, wherein the redistributing at least a portion of the electroactive substance comprises: removing a portion of the electroactive substance from the coated carbon nanotubes coated with the electroactive substance into a solution, and depositing the electroactive substance from the solution onto surfaces of the uncoated carbon nanotubes.
 5. The method of claim 1, wherein the redistributing at least a portion of electroactive substance comprises removing electroactive substance from the coated carbon nanotubes and placing the removed electroactive substance on the uncoated carbon nanotubes.
 6. The method of claim 1, wherein the redistributing of the at least a portion of the electroactive substance comprises: providing to the electrically conductive network a solvent in which the electroactive substance has a solubility of less than 1 g/100 g, but greater than zero; dissolving at least a portion of the electroactive substance into the solvent; and depositing the electroactive substance from the solution to the uncoated carbon nanotubes within the electrically conductive network.
 7. The method of claim 6, wherein the redistributing at least a portion of the electroactive substance comprises: subjecting the electrically conductive network to electrical charge and discharge in the solvent to dissolve at least a portion of the electroactive substance from the coated carbon nanotubes coated with the electroactive substance into the solvent, then depositing the dissolved electroactive substance from the solvent onto the uncoated carbon nanotubes.
 8. An electrically conductive network, comprising an active material coating; first carbon nanotubes coated with the active material coating; and second carbon nanotubes partially coated with the active material coating from the first carbon nanotubes, wherein at least a portion of the surfaces of the second carbon nanotubes directly contact surfaces of other second carbon nanotubes without the active material coating between these second carbon nanotubes, and wherein the first carbon nanotubes and the second carbon nanotubes are entangled to form an electrically conductive network.
 9. The electrically conductive network of claim 8, wherein the electrically conductive network further comprises interstitial spaces between the first carbon nanotubes and the second carbon nanotubes, and wherein the active material coating is located on surfaces of the first carbon nanotubes and surfaces of the second carbon nanotubes within the interstitial spaces.
 10. The electrically conductive network of claim 9, wherein the interstitial spaces comprise spaces with a maximum dimension of less than 200 nm.
 11. The electrically conductive network of claim 8, wherein the first carbon nanotubes and the second carbon nanotubes comprise at most 50% by weight of the electrically conductive network.
 12. The electrically conductive network of claim 8, wherein the active material coating comprises at least 70% by weight of the electrically conductive network.
 13. The electrically conductive network of claim 8, wherein the electrically conductive network has a volume of porosity of 50 to 90 vol. %.
 14. The electrically conductive network of claim 8, wherein the active material coating comprises a compound of Ni, Zn, Cd, Fe, Pb, Mn, Co, Ag, Al, or Mg.
 15. The electrically conductive network of claim 8, wherein the active material coating comprises a compound of Ni or Zn. 